PEEP is a standard treatment for the care of patients undergoing mechanical ventilation. PEEP consists of applying a constant positive pressure to the patient's airways during expiration, with the aim of avoiding expiratory lung collapse. Application of PEEP is particularly important during mechanical ventilation of infants demonstrating lower than adequate End-Expiratory Lung Volumes (EELV). The combination of a highly compliant chest wall, and a small caliber of intrathoracic airways predispose infants to closure of the small airways, alveolar collapse at the end of expiration, and thus predispose to hypoxemia [Findley, L. J.; Ries, A. L.; Tisi, M.; Wagner, P. D.; “Hypoxemia During Apnea in Normal Subjects: Mechanisms and Impact of Lung Volume”; J. Appl. Physiol.; December 1983; 55(6); pp 1777-83].
Continuous Positive Airway Pressure (CPAP) is a continuous delivery of positive airway pressure to the patient during both inspiration and expiration, while Biphasic Positive Airway Pressure (BiPAP) is a biphasic delivery of positive airway pressure to the patient.
A variety of different interfaces (nasal prongs, face mask, or endotracheal tube, etc.) are currently used to deliver CPAP to a patient's airways via either continuous or variable gas flow ventilators. CPAP is sometimes also called Continuous Distending Pressure (CDP). The function of CPAP is to keep the patient's airways open.
BiPAP can be described as pressure-controlled ventilation in a system allowing unrestricted spontaneous breathing at any moment of the ventilatory cycle. It can also be described as a CPAP system with a time-cycled change of the level of applied CPAP. BiPAP is not necessarily time-cycled; it can be triggered by the patient. As with a pressure controlled, time-cycled mode, the duration of each phase (T(high), T(low)) as well as the corresponding pressure levels (P(high), P(low)) can be adjusted independently from each other.
Normally, the vagally-mediated Hering-Breuer (H-B) reflexes play a role in the maintenance of an adequate EELV. It is currently believed that, in mammals, phasic stimulation of slowly adapting pulmonary stretch receptors located throughout the tracheobronchial tree and lungs, as would occur with each breath, modulates the frequency of breathing by inhibiting further inspiration during the inspiratory phase or causing prolongation of expiration during the expiratory phase; this is the Hering-Breuer reflex. Tonic stimulation of the same receptors, or other receptors, will also occur with changes in residual lung volume (or Functional Residual Capacity (FRC)) to alter the timing of inspiration relative to expiration in a manner that acts to stabilize the residual lung volume. There may be other reflexes involved, but to date this is the most frequently described reflex. The mechanisms involved with regulating EELV comprise:
1. Braking of Expiratory Flow:
                It has been suggested that changes in a patient's upper airway resistance “brakes” expiration to EELV in order to promote an increase in EELV [Kosch, P. C.; Stark, A. R.; “Dynamic Maintenance of End-Expiratory Lung Volume in Full-Term Infants”; J. Appl. Physiol.; October 1984; 57(4); pp 1126-33]. It has also been claimed that EELV is maintained by post-inspiratory EAdi (Electrical Activity of the diaphragm) [Mortola, J. P.; Fisher, J. T.; Smith, B.; Fox, G.; Weaks, S.; “Dynamics of Breathing in Infants”; J. Appl. Physiol.; May 1982; 52(5); pp 1209-15].2. Tonic EAdi:        The article [Lopes, J.; Muller, N. L.; Bryan, M. H.; Bryan, A. C.; “Importance of Inspiratory Muscle Tone in Maintenance of FRC in the Newborn”; J. Appl. Physiol.; October 1981; 51(4); 830-4] points out the contribution of tonic EAdi (persistence of EAdi throughout the entire expiration) in the maintenance of EELV.3. Respiratory Timing:        A decrease in FRC reflexly increases Te, which increases EELV by causing intrinsic PEEP. As indicated in the foregoing description, FRC is the Functional Residual Capacity, i.e. the volume of air in the lungs at the end of passive expiration. Te is the neural expiratory time defined as the time between the end of neural inspiration and the beginning of the subsequent neural inspiration.        
The expiratory braking mechanism is non-functional in intubated, mechanically ventilated infants, because the endotracheal tube maintains the infant's upper airways patent, albeit the presence of activation of the upper airway muscles. This interferes with the maintenance of EELV in intubated babies; the intubated babies must rely on the remaining two mechanisms ((2) Tonic EAdi (3) Respiratory timing) to maintain their EELV. Externally applied PEEP is almost always used in intubated babies with the intention of maintaining EELV; however, the level of PEEP to be applied is still determined in a more or less arbitrary fashion.
The presence of tonic EAdi in mechanically ventilated infants has been previously demonstrated and quantified [Emeriaud, G.; Beck, J. C.; Tucci, M.; Lacroix, J.; Sinderby, C.; “Diaphragm Electrical Activity during Expiration in Mechanically Ventilated Infants; 2003 ATS Abstract]. The diaphragm remains partially active during expiration, and is characterized by an initial period of post-inspiratory EAdi followed by tonic EAdi. The mean value of this tonic EAdi was 12-14% of the inspiratory Eadi. However, as can be seen in appended FIG. 1, this end-expiratory tonic EAdi is quite variable.